Isotactic polypropylene produced by using a stereo-regular catalyst is widely used as a material from which various shaped articles are made because of its high stiffness, strength, moldability, and heat resistance, and because it provides a shaped article having good appearance. A polypropylene film is highly evaluated for its transparency and stiffness and is currently used as a packaging material for various objects.
One defect of the polypropylene is its low low-temperature resistance, i.e., its impact strength depends upon temperature so greatly that the impact strength is greatly reduced from room temperature to 0.degree. C. Another defect is that the temperature at which a polypropylene film can be heat-sealed or at which a stretched film contracts is so high that, if, for example, a biaxially stretched film is heat-sealed at a temperature which adequate heat sealing can be achieved, the film contracts with heat and its appearance is impaired. Therefore, the heat sealing of a polypropylene film is practically impossible. The hot tack property is one of the most important requirements that films must satisfy. Many types of film are used to pack a wide variety of objects such as foods (e.g., potato chips, rice crackers, biscuits, sugar, rice, wheat, wheat flour and other cereals), as well as industrial products (e.g., bolts, nuts, nails and screws). Since the heat-sealed edges of the packaging film are placed under the weight of the contents before the edges are cooled adequately, it often occurs that the edges break or a pinhole is found to be formed therein. This means a film having good heat sealing property is not suitable for practical use unless its hot tack property is also good.
In order to overcome these defects of polypropylene, propylene random copolymers such as propylene-ethylene copolymer, propylene-butene-1 copolymer and propylene-ethylene-butene-1 copolymer have been produced by copolymerizing propylene with a small amount of ethylene and .alpha.-olefins such as butene-1. These copolymers either alone or in combination with other resins or rubber are used in the heat-sealable layer of a biaxially stretched polypropylene film, a shrink-pack film, or a low-temperature resistant polypropylene film. These propylene copolymers are generally produced by slurry polymerization using an inert solvent such as hexane or heptane (such polymerization being hereunder referred to as "solvent polymerization"). However, in many cases, this method of polymerization produces a valueless amorphous polymer which dissolves in the solvent, and causes a great loss in the monomers which is economically disadvantageous. Furthermore, an increase in the viscosity of the slurry requires more power to achieve the same degree of agitation and the efficiency of heat transfer in the polymerization vessel is decreased, and all these phenomena reduce the productivity of the solvent polymerization. These defects are particularly conspicuous when a propylene copolymer of increased comonomer content (e.g., ethylene and an .alpha.-olefin such as butene-1) having improved hot tack property and heat sealing property is to be produced by the solvent polymerization.
The prior art propylene copolymers have serious defects in physical properties. The propylene-ethylene copolymer does not have satisfactory heat sealing property, stiffness, slipperiness and blocking property. The propylene-.alpha.-olefin (e.g., butene-1) copolymer is very low in hot tack property, and its low-temperature resistance, i.e., impact strength at low temperature, is poor. The propylene-ethylene-butene-1 copolymer has somewhat improved heat sealing property but is low in hot tack property and stiffness, and the balance of the physical properties in processing into a film of the copolymer is far from being satisfactory. A block copolymer of propylene or ethylene or an .alpha.-olefin is known and has improved low-temperature resistance without undergoing much decrease in stiffness. But, such block copolymer is poor in transparency, and in particular, the transparency of a film of the copolymer is extremely low and no reduction in the temperature at which the film can be heat-sealed is achieved.
In view of the above, there has not yet been found a propylene copolymer which has improved hot tack property and heat-sealing property as well as a good balance of the other physical properties and which can be produced consistently on an industrial scale.